The present invention relates to rotating disk storage devices such as a magnetic disk drive and a magneto-optical disk unit, and more particularly to a rotating disk storage device having a function of restoring a slider to a normal flying position if the slider is flying stably at a position higher than a normal position above a disk.
In a magnetic disk drive, a head/slider moves on a surface of a rotating magnetic disk while flying with the extremely slight height being kept from the surface. The head/slider is positioned at a given track where data is read or written. The head/slider floating off the surface of the magnetic disk is supported by a spring structure, which is called a flexure, from a load beam constituting a head suspension assembly (hereinafter referred to as HSA). The head/slider keeps a given flying height by pivotally moving about a dimple formed in a flexure tongue.
The pivoting motion is in general known as the pitch and roll motion or the gimbal motion. Fluctuations in buoyant force applied to an air bearing surface (hereinafter referred to as ABS) of the slider, and irregularity of the behavior of the HSA, and so on, continuously cause fluctuations in tracking position even after the head/slider is aligned to a track. The pivoting motion, however, gives the slight pitching motion and the slight rolling motion to the slider so as to compensate for the fluctuations in tracking position.
Here, the rolling motion is motion in which the slider positioned in such a manner that the angle between the ABS and the disk surface becomes constant pivotally moves about an X axis which is assumed to be provided in the longitudinal direction of the slider. The pitching motion is motion in which the slider pivotally moves about a Y axis which is assumed to be provided in the lateral direction of the slider. In addition, the pressing load with which a load beam presses the head/slider onto the surface of the magnetic disk also influences the flying height of the slider. The characteristics of the pivoting motion depend on pitch-stiffness, roll-stiffness and pressing load. Each component of the HSA has specified tolerances so that these parameters are kept within given ranges.
In a magnetic disk drive with a ramp, which uses the load/unload method, when a head starts reading or writing, the head/slider which is being retracted into the ramp is moved to fly above a surface of a magnetic disk. This movement is called loading. The ramp is provided in an area that is outside and close to the circumference of the magnetic disk. At the time of loading, the head/slider is first positioned in the vicinity of the outermost circumferential track of the magnetic disk.
Next, when the head/slider is loaded, the head/slider moves to a tip of a sloping surface of the ramp from the home position. As a result, from the point of time at which the head/slider does not completely flies above the surface of the magnetic disk, airflow on the surface of the magnetic disk begins to influence the head/slider, which may cause the head/slider to oscillate. Accordingly, the posture or behavior of the head/slider becomes unstable at the moment of getting away from the tip of the ramp to completely fly above the surface of the magnetic disk, which may result in an abnormal condition of the flying height. If the above-mentioned tolerances of the HSA's components are severely specified, the posture or behavior of the head/slider rarely becomes unstable. However, if the tolerances are specified more severely than necessary, a problem arises in which production yields will decrease.
Japanese Patent Laid-Open No. 2001-229637 discloses the technology for detecting an abnormal condition of the momentary flying height during track following. When write operation is started, then on the basis of the top (hereinafter referred to as preamble) of servo data read by an automatic gain controller (hereinafter referred to as AGC), a gain of the servo signal is determined to amplify a read signal of the following servo data. Next, the gain of the AGC stored in a register of a channel controller is compared with its reference value to detect an abnormal condition of the flying height of a magnetic head.
Japanese Patent Laid-Open No. 2002-100139 discloses the technology in which immediately after the head/slider is loaded above the magnetic disk from the ramp, the back EMF or BEMF (Back Electromotive Force) control is performed until it is judged that the head/slider enters in a state in which servo data can be read stably and performing the seek control does not produce any problem. In the disclosed back EMF control, immediately after loading the head/slider from the ramp, a servo channel is opened to read servo data. However, the servo channel is not used for the seek control of the HSA. Instead, an output of a back EMF monitor is used to control the operation of the HSA. There is disclosed the technology in which at the point of time at which it is judged that the servo data could be properly read, a head is positioned in an area other than an effective recording area, which corresponds to the circumferential area of a magnetic disk. Then the track following is performed, for example, for a period of 40 msec to blow off contaminations adhered to the head by the wind pressure produced by the disk rotation. Moreover, the reference also discloses the technology in which on the basis of the knowledge that flying of the head is not stable for a fixed period of time after the head is loaded from the ramp, verification is executed for write operation performed for the fixed period of time immediately after the head is loaded.
Japanese Patent Laid-Open No. 2000-132933 discloses the technology in which a state of a magnetic head is always monitored by use of an amplitude value of an output signal of the magnetic head. An amplitude value of an output signal of the magnetic head in a servo area is determined and compared with a reference amplitude value of an output signal of a normal magnetic head. If the determined amplitude value is smaller than the reference amplitude value, then it is judged that an abnormal condition has occurred. In the event of an abnormal condition, writing of data to the next data track of a servo pattern is canceled.
Since the pivoting motion of a head/slider is subtle, it is difficult to precisely keep track of its behavior. If the head/slider stably flies at a position higher than a normal position, it is detected as a phenomenon such as a seek error, a data read error, or a data write error. However, even if these error phenomena are detected, it is not easy to immediately identify an abnormal condition of the flying height of the head/slider as its cause from among various kinds of possible factors. In particular, if the flying height is at a position between the normal height and the height from which data cannot be read at all, a problem is produced. In this case, although servo data can be read by correcting it, there is a possibility that writing of data will fail. Therefore, in order to completely prevent such an error from occurring, it is necessary to verify all write operation. The verification, however, will cause the performance of the drive to decrease more than necessary.